First get this book: "Second Hand Dog" by Carole Lea Benjamin. It's super. Easy to read and full of her cartoons, it makes great though-provoking points.
You might try to schedule walks with this dog when no one else is out for a while. Socialization of this sort has to begin slowly -- because if routine walks become battlezones then you have more de-sensitzing to do.
Arrange to meet some friends -- with and without dogs. Set it up BEFORE -- at first don't even approach, but when you see them in the distance, put up a hand and have them NOT approach, but rather you take the dog and sit it and reward for calm behavior. Then gradually decrease the distance between you and a 'stranger' and another dog. But keep it in very very short, successful trips. Always quit while you are ahead. A tired frazzled dog can't 'win' and will be over-wrought.
Search for ways to get exercise without encountering scads of others -- and keep all encounters brief and controlled. I know this is hard, but for THIS dog it is important to take the pressure off. Every encounter is probably 'wild' for him -- a totally new experience that may be scarey.
This baby has been kept in a box all his life, so even things like the steps at the library are 'new' to this dog. Flapping laundry in the breeze is a scarey thing, and even a big old ball rolling around is probably heart-stopping.
So plan little jaunts and actually set up obstacles for the dog, and plan on times that aren't busy. Approaching a set of swings at a park when there are NO KIDS around may sound tame to you, but for this baby it will be scarey til he gets to smell and know it's not all out to get him.
Go from inanimate objects to real people and then real dogs and other animals. Be honest with people -- smile sweetly and say "My dog is a rescue and he's had a tough life ... so right now we're in some serious training so honestly I'm going to cross the street and go around you -- not because I think you are a bad person, but just because I'm trying to keep Poochie here calm"
By talking to the person (who may be 30 feet from you) while you put Poochie in a 'sit' and then praise him for any calmness you can get in behavior helps HIM see you are in control. Trust has to be a big huge issue for him.
Eventually you will be able to carry treats with you and when you meet someone you KNOW (I never like strangers giving MY dogs treats -- only from friends I give them to), then you can hand them a goodie and ask them to hand it to Poochie palm flat.
Be patient -- it took some idiot many years to scare and ill-prepare this dog for life. It's gonna take you MONTHS to make any headway at all. But you will. If you are patient and break this down into teeny weeny tiny steps it WILL work.
Make sure you warn your trainer -- be honest. But a good trainer will help you set this dog up to succeed.
This is gonna be a lot of work, but you will feel an incredible sense of accomplishment with every step. He may not make obedience champ of the year ... but then again, he might!!
Wed this dog to you emotionally -- praise him for calm behavior IN the house. Start basic obedience in the house -- you can teach 'sit' anywhere. And it will be a bigger accomplishment for THIS dog than any other.
Try this. Just sit in a chair calmly with a treat in each hand. In fact with a dog this size, sit on the floor. Just kind of ignore him ... when he is in front of you calmly speak his name and the instant he looks AT you, shove that treat in his mouth. It's gotta be *fast*.
The object of this is to get his attention with your voice instantly. It won't take long -- but you want him to associate you speaking his name with eye contact and GOOD THINGS. "When I say your name, you look at me and the world is grand!!
That will help you everywhere -- wean off the treats and substitute effusive, enthusiastic praise (a verbal PARTY). But that's gonna make it 100 times easier in class if he has learned to look AT you and pay attention to you when you speak to him.
I realize this is a far cry from walking on leash -- but you can't get a dog to obey on leash if you can't get their attention. And for a dog like this, learning to please you is gonna be a big thing. In fact, *wanting* to please you may be the biggest thing of all because I'd bet human "attention" hasn't always been a good or desirable thing.